Mobile phone keypads require a means of backlighting so the user can identify the individual key. A common means of lighting the keypad is by using LEDs. An alternative means of backlighting the keypads is by an electroluminescent (EL) lamp. The advantages of an EL lamp versus LEDs are its physical thinness, physical flexibility, and light uniformity. The disadvantage of EL lamps is that they produce audible noise. The mobile phone manufacturers are always challenged to produce thinner and thinner phones. For this reason, EL lamps have been chosen as a means to light up the keypad without any substantial increase in overall thickness.
The EL lamps flexibility and thinness allows the EL lamp to be placed just underneath the keypad. The EL lamp's light uniformity eliminates the need for a light diffuser. The uniform light provides the keypad with a more pleasing appearance. The construction of the lighted keypad can therefore be made very thin. Being that the end product is a mobile phone; any audible noise generated by the EL lamp is undesirable. Care must be taken to minimize electrical coupling to the keypad sensing circuit.
Therefore, a need exists to provide a device and method to overcome the above problems. The device and method will provide a circuit and method of minimizing the audible noise generated by the EL lamp and electrical noise to the keypad circuitry. A means of minimizing the audible noise by special drive techniques will allows mobile phone manufacturers to employ EL lamps to produce thin lighted keypads for their final product.